In known fuel cell systems employing phosphoric acid electrolyte or a high temperature polymer electrolyte membrane (HTPEM), cooler plates interposed between groups of fuel cells have a simple serpentine cooler flow path and utilize water coolant. Liquid water enters the cooler plates and a two-phase, water/steam mixture exits the cooler plates. A small fraction of the heat removal is due to increasing the sensible heat of the water as it increases to its boiling temperature, and a major fraction of heat removal is due to the latent heat of evaporation of liquid water to steam. U.S. Pat. No. 3,969,145 describes such a coolant system.
In any phosphoric acid fuel cell, the useful life of the fuel cell is determined principally by the rate at which phosphoric acid evaporates into the reactant gases and is not condensed back to a liquid before exiting the fuel cells. Non-reactive acid condensation zones at the reactant gas exits of the fuel cells minimize acid loss due to evaporation and thereby maximize life of the fuel cell stack. Such condensation zones are taught in U.S. Pat. Nos. 4,345,008 and 4,414,291, and in PCT patent publication WO 00/36680. The condensation zones should be below 140° C. (280° F.) in order to assure sufficient condensation of electrolyte so that the fuel cell stack will perform for at least ten years, which in turn requires that the coolant inlet temperature must be less than 140° C. (280° F.) in prior systems.
A competing problem in a phosphoric acid fuel cell stack is that the fuel processing system, such as a steam reforming fuel processor, converts various hydrocarbon fuels to hydrogen-rich reformate which contains between 0.3% and 1.0% carbon monoxide (CO), which is a poison to the anode catalyst and impedes the oxidation of hydrogen at the anode. The extent of poisoning is a function of the concentration of CO and cell temperature. At the likely concentrations of CO referred to hereinbefore, the temperature within the electrochemically active portion of each cell must be kept above 150° C. (300° F.) in order to provide reliable fuel cell performance. Thus, the temperature suited for condensation is lower than the temperature required for CO tolerance.
In patent publication US 2006/0141312, coolant inlets are adjacent the non-reactive zones of the fuel cells, the resulting low temperatures promoting condensation of electrolyte which has evaporated into the reactant gases of the fuel cells. Coolant in a second zone, adjacent to the non-reactive zone, flows generally toward the non-reactive zone assuring the edge of the reactive zone adjacent to the non-reactive zone will be at a temperature high enough to mitigate CO poisoning of the catalysts.
Referring to FIG. 1, a fuel cell stack 9 of said publication includes a plurality of fuel cells 10 having three zones 11-13. Each of the zones 11-13 generally overlaps one of three fuel flow passes in the fuel flow channels. Fuel enters through a fuel inlet manifold 17, and flows to the right (as seen in FIG. 1) through the fuel flow fields associated with the third zones 13. Then the fuel flows through a turnaround manifold 19 and then to the left (as seen in FIG. 1) through the fuel flow fields associated with the second zones 12, to a second turnaround manifold 20, and then to the right (as seen in FIG. 1) through the flow fields associated with the first, non-reactive zones 11, and outwardly through a fuel exit manifold 22.
The first zones 11 are non-reactive because the portion of each fuel cell comparable with the first zones 11 does not have a cathode catalyst and therefore will not react with the reactant gases.
In the embodiment herein, the oxidant reactant gas, such as air, flows into an air inlet manifold 25 and then flows downwardly (as seen in FIG. 1) through the oxidant reactant gas flow fields in the third zones 13, the second zones 12, and the first zones 11, and thence outwardly through an air outlet manifold 26.
In FIG. 2, the fuel cell stack 9 planform configuration is shown as in FIG. 1, but in addition, includes the pattern of coolant flow passageways, such as tubing or channels, in cooler plates, which typically are placed between groups of several fuel cells, throughout the fuel cell stack. For instance, in a 200 kW fuel cell system, there may be 35 coolers, placed between each group of eight fuel cells in a stack of 272 fuel cells. The cooler plates are fed through external coolant manifolds, including a coolant inlet manifold 29 and a coolant outlet manifold 30.
From the inlet manifold 29, the coolant flows adjacent the first, non-reactive zones 11 to the left and then to the right through coolant flow passageway segments 30, 31 respectively. Thus, the coolest coolant is provided adjacent non-reactive zones so as to cause significant condensation of electrolyte which may have evaporated into the reactant gases, as the reactant gases flow out of each of the fuel cells, without the cool temperature supporting CO poisoning of catalyst.
The coolant then flows through segments 33 of the coolant flow passageways adjacent the first zones 11, the second zones 12 and the far side (to the right in FIG. 3) of the third zones 13. The coolant then flows through segments 34 of the coolant flow passageways adjacent the third zones 13 and the second zones 12.
A “substack” is a group of cells between two cooler plates. The center cells within a substack are the hottest and the cells adjacent the coolers are the coolest. Acid loss is proportional to the local temperature at the exit of each pass of fuel or air. Extensions 33-34 lower the local temperature at the exit of the first fuel pass and thus reduce acid loss into the first fuel pass relative to a cooler plate design that does not have extensions 33-34. The fuel reactant of the several cells of a substack are well mixed together in fuel turn manifold 19. This results in all cells within the substack receiving a uniform quantity of acid. The hot cells receive less acid than they lost and the cold cells receive more acid than they lost.
The coolant then flows through serpentine flow segments 37-41 in a direction which is parallel with the direction of the oxidant reactant gas flow, from top to bottom as seen in FIG. 1, adjacent the second zones 12. This results in a reactive zone temperature adjacent to the non-reactive, condensation zones, that is above 150° C. (300° F.) which substantially reduces CO poisoning of the anodes. Stated alternatively, the flows adjacent the first, non-reactive, condensation zones 11 and the flows adjacent the second zones 12 are such as to provide a sharp temperature gradient at the interface between the zones 11 and 12, so that the reactive portion of the fuel cell is well above 150° C. (300° F.) to avoid CO poisoning, while condensation will occur in the non-reactive zone at a temperature below 140° C. (280° F.).
The coolant then flows in a serpentine fashion through a plurality of segments 45-50 in a direction which is generally opposite to the flow of oxidant adjacent the third zones 13, to the coolant outlet manifold 30.
If desired, coolant flow channels may be established so that segment 33 joins directly with segment 50, as shown in said publication, and coolant flow in the third zone 13 will be toward the first zone 11. The coolant exit manifold 30 may then be to the left of the air inlet manifold 25.
Alternative fuel and air configurations, modified polybenzimidazole (PBI) membranes, polymer membranes based on polyazoles, polyphosphoric acid and free acid electrolytes, may be used, as in said publication. Single phase coolant, such as water, or dual phase coolant, such as a water/steam mixture may be employed.
The problem with this design is that in order to provide cooling to that portion of the fuel flow just before it enters the turnaround manifold 19, adjacent the segments 33a (FIG. 3) and 34 the region adjacent the segments 33b, shown by hatch lines 60 in FIG. 3, is cooled below the temperature at which sensitivity to CO poisoning is very high. This may be in the range of 150° C. (300° F.) to 165° C. (325° F.). This results from holding the coolant temperature required in the condensation zone 11 sufficiently low for adequate condensation, and the fact that there is relatively little heat generation in the region 60.